Glucocorticoids inhibit the activity of the hypothalamo-pituitary- adrenocortical (HPA) axis by inhibiting the synthesis and release of hypothalamic factors which stimulate the anterior pituitary. When glucocorticoid feedback inhibition is impaired, the glucocorticoid response to stress is prolonged. Chronic elevations in plasma glucocorticoid levels result in a reduction in corticosterone binding in the hippocampus and in hypersecretion of glucocorticoids in response to stress. This finding is analogous to the aging rat, in which there is a hypersecretion of glucocorticoids in response to stress and an age-related loss of hippocampal neurons in areas of high corticosterone binding capacity. While previous studies predict that glucocorticoid feedback inhibition of the HPA axis is exerted at the level of the hippocampus, there is also functional and anatomical evidence to suggest that glucocorticoids have inhibitory effects directly on the synthesis and release of hypothalamic factors stimulatory to the HPA axis. The purpose of the proposed studies is to determine the relative importance of glucocorticoid feedback at the level of the hypothalamus in the regulation of basal and stress-stimulated HPA axis activity. As an index of HPA axis activity, measurements will be made of the nuclear precursor RNA and nuclear and cytoplasmic mRNA coding for corticotropin-releasing factor (CRF), the most potent of the hypothalamic ACTH-releasing neurohormones. These measurements will be compared with determinations of CRF peptide secretion in vitro, or with ACTH and corticosterone secretion in vivo. The following specific aims are proposed: Specific Aim 1: Determine the extent to which corticosterone inhibits basal and stimulated CRF peptide secretion and gene expression by direct effects at the level of the hypothalamus. Specific Aim 2: Determine the extent to which hypothalamic versus extrahypothalamic glucocorticoid feedback regulates the basal levels of CRF biosynthesis in the PVN in intact and fornix-transected rats. Specific Aim 3: Determine the effect of the removal of extrahypothalamic input on feedback inhibition of stress-stimulated CRF gene expression using an adrenalectomized/steroid replaced model in intact and fornix-transected rats. These studies should provide new insight into the relative importance of the direct role of the CRF neuron in glucocorticoid feedback regulation of the stress response.